Exchange Surfaces

Exchange Surfaces

For exchange to be effective:

  • The surface area of an organism must be large compared with its volume.
  • Thin exchange surface to give a short diffusion pathway
  • Partially permeable to allow selected materials to cross
  • Movement of the environmental medium (e.g. air)
  • Movement of the internal medium (e.g. blood)

Gas exchange in single-celled organisms

Single-celled organisms have a large surface area to volume ratio so oxygen can be absorbed by diffusion across their body surface, which is covered by only a cell-surface membrane. Carbon dioxide from respiration diffuses out across their body surface in the same way.

Gas exchange in insects

Insects must balance the opposing needs of exchanging respiratory gases with reducing water loss (which occurs easily for terrestrial organisms).

To reduce water loss, terrestrial organisms have waterproof coverings and a small surface area to volume ratio to minimise the area over which water is lost. These features mean that insects cannot use their body surface to diffuse respiratory gases in the same way a single-celled organism does. Insread, they have developed an internal network of tubes called trachae, which are supported by strengthened rings to prevent them from collapsing.  These then divide into smaller tubes called tracheoles which extend throughout all the body tissues of the insect. In this way, oxygen can be brought directly to respiring tissues.

Respiratory gases move in and out of the tracheal system via a diffusion gradient (oxygen is used up in respiration so its concentration towards the end of the tracheoles falls) and ventilation (movement of muscles in insects can create mass movements of air in and out of the trachae).

Gases enter and leave trachae through tiny pores called spiracles, on the body’s surface. The spiracles may be opened and closed by a valve. When they are open, water can evaporate from the insect.

The tracheal system relies mostly on diffusion to exchange gases between the environment and cells. For diffusion to be effective the pathway must be short, as a result, this limits the size that insects can attain.

Gas exchange in fish

Fish have developed a specialised internal gas exchange surface (gills) because their waterproof covering and small surface area to volume ratio means their body surface is not adequate to supply and remove their respiratory gases via diffusion.

Gills are located behind the heads of fish and are made up of gill filaments which are stacked up in a pile. At right angles to te filaments are gill lamellae which increase the surface area of the gills.

Water is taken in through the mouth and forced over the gills and out through and opening on each side of the body. The flow of water over the gill lamellae and the flow of blood within them are in opposite directions (countercurrent flow). This is so that there is always a higher concentration of oxygen in the water than in the blood, so it diffuses into the blood along the whole length of the lamellae.



Gas exchange in a leaf

Circulatory system of a mammal

Features of transport systems:

  • A suitable medium in which to carry materials (e.g. blood)
  • A form of mass transport in which the transport medium is moved around in bulk
  • A cosed system of tubular vessels that contains the transport medium and forms a branching network to distribute it to all parts of the organism

  • A mechanism for moving the transport medium within vessels (e.g. muscular contraction)
  • A mechanism to maintain the mass flow movement in one direction (e.g. valves)
  • A means of controlling the flow of the transport medium to suit the needs of different parts of the organism

Artery structure related to function:

  • Thick muscle layer – can constrict and dilate to control the volume of blood passing through them
  • Thick elastic layer – stretching and recoil helps maintain high blood pressure and smooth the pressure surges created by the beating of the heart
  • Overall thickness – helps prevent vessel bursting under pressure
  • No valves – blood is under constant high pressure so does not tend to flow backwards


Tissue fluid and its formation

Tissue fluid is a watery liquid that contains dissolved oxygen and nutrients. It supplies these necessary solutes to the tissues and receives waste materials such as carbon dioxide in return. It is therefore the means by which materials are exchanged between blood and cells, and as such, it bathes all the cells of the body. It provides a mostly constant environment for the cells it surrounds.

Blood is pumped along arteries, into narrower arterioles and then narrower capillaries, creating hydrostatic pressure at the arterial end of the capillaries. This pressure forces tissue fluid out of the blood plasma however this pressure is opposed by two other forces:

  • Hydrostatic pressure of the tissue fluid outside the capillaries
  • Lower water potential of the blood, due to plasma proteins, pulling water back into the blood within the capillaries

The combined effect of these forces is to create an overall pressure that pushes tissue fluid out of the capillaries. This pressure is only enough to force small molecules out of the capillaries, leaving all cells and proteins in the blood. This type of filtration under pressure is known as ultrafiltration.

Return of tissue fluid to the circulatory system:
Once it has exchanged metabolic materials with the cells it bathes, tissue fluid must be returned to the circulatory system. Most tissue fluid returns to the blood plasma directly via the capillaries since the hydrostatic pressure within the capillaries has been reduced due to the loss of tissue fluid, as a result, by the time the blood has reached the venous end, its hydrostatic pressure is less than that of the tissue fluid outside it. In addition, the osmotic forces resulting from he proteins in the blood plasma pull water back into the capillaries.

The remainder of the tissue fluid is carried back via the lymphatic system.

Movement through the roots

Roots are composed of different tissues each with their own function.


  • Epidermis – a single layer of cells often with long extentions called root hairs which increase the surface area. A single plant may have 1010 root hairs.
  • Cortex – a thick layer of packing cells often containing stored starch.
  • Endodermis – a single layer of cells that surround the vascular tissue containing a waterproof layer called the casparian strip which allows the plant to control the movement of ions into the xylem.
  • Pericycle – a layer of undifferentiated meristematic (growing) cells.
  • Vascular tissue – this contains xylem and ploem cells which are continuous with the stem vascular bundles.

Water moves into the root hair cells by osmosis since there is a lower water potential in the cell than in the soil. The root hair cells are efficient surfaces for exchange because they provide a large surface area as they are long extentions and they occur in thousands on each root. They also have a thin cell wall and cell membrane so give a short osmotic pathway.

Water moves through the root via two pathways: the symplastic pathway and the apoplastic pathway.

The symplast pathway

This consists of the living cytoplasms of the cells in the root. Water is absorbed into the root hairs by osmosis since the cells have a lower water potential than the water in the soil. Water then diffuses from the epidermis through the root to the xylem, down a water potential gradient. The cytoplasms of all the cells in the root are connected by plasodesmata through holes in the cell walls, so there are no further membranes to cross until the water reaches the xylem, and so no further osmosis.

The apoplast pathway

This consists of cell walls between cells. The cells walls are quite thick and very open so water can simply diffuse through cell walls down the water potential gradient. There are no cell membranes to cross so it moved by diffusion, not osmosis. However, the apoplast pathway stops at the endodermis because of the waterproof casparian strip, which seals the cell walls. At this point water has to cross the cell membrane by osmosis and enter the symplast. This allows the plant to have some control over the uptake of water into the xylem. Around 90% of water transport through the root uses the apoplast pathway, as the available volume is greater.

The uptake of water by osmosis actually produces a force that pushes water up the xylem. This force is called root pressure which can be measured by placing a manometer over a cut stem. This force helps push water up short stems i.e. a few centimetres however longer distances like up trees would require a much greater pressure.

Movement through the stem (mass flow):

The xylem vessels form continuous pipes from the roots to the leaves. Water can move up through these pipes to a height of over 100m. Since the xylem vessels are dead, open tubes, no osmosis can occur within them, thus water moves by mass flow. The driving force for the movement is transpiration in the leaves. This causes low pressure in the leaves, so water is drawn up the stem, replacing the lost water. The column of water in the xylem vessels is therefore under tension. Fortunately, water has a high tensile strength due to the tendancy of water molecules to stick together by hydrogen bonding (cohesive), so the water column does not break under the tension. This mechanism of pulling water up a stem is sometimes called the cohesion-tension mechanism.

Root pressure also pushes water up from beneath. It arises because mineral ions are actively taken up into the xylem in the root. If transpiration is slow then these ions are not transported up the stem, they build up in the root xylem. This lowers the water potential in the root tissue and water is drawn into the root by osmosis, pushing the column of water upwards.

Movement through the leaves:

The xylem vessels ramify (branch) in the leaves to form a system of fine vessels called leaf veins. Water diffuses from the xylem vessels in the veins through the adjacent cells down its water potential gradient. As in the roots, it uses the symplast pathway through the living cytoplasm and the apoplast pathway through the non-living cell walls. Water evaporates from the spongy cells into the sub-stomatal air space and diffuses out through the stomata.

Each stomata is surrounded by guard cells which inlike the rest of the epidermal cells, contain chloroplasts which allow them to photosynthesise and produce ATP, which they use to drive active transport ion pumps, which mean they can qickly alter their water potential.

To open the stomata the guard cells pump ions into the cell which lowers the water potential so water enters by osmosis. The cells becoem turgid and bend apart so the stoma between them opens.

To close the stomata the guard cells pump ions out of the cell, which raises their water potential so water leaves by osmosis. The cells become flaccid and striaghten to the stoma between them closes.

Evaporation of water is an endothermic process since energy must be put in to turn water from a liquid to a gas. This energy is provided by the sun, in a process separate to that by which is provides light energy for photosynthesis.

Factors affecting transpiration

Temperature – high temperature increases the rate of evaporation of water from the surface of the spongy mesophyll cells because it increases the kinetic energy of the water molecules. This raises the Ψ in the sub-stomatal air space and means that the molecules are moving faster so tanspiration increases.

Humidity – high humidity means a higher Ψ in the air surrounding the stomata, so a lower Ψ gradient between the sub-stomatal air space and the eair outside,so less evaporation.

Air movement – wind blows away saturated air from around the stomata, replacing it with drier air with a lower Ψ , so increasing the Ψ gradient and increasing transpiration.

Light intensity – light stimulates plants to open their stomata to allow gas exchange for photosynthesis, which also increases the rate of transpiration as a side effect.

If plants are losing too much water and their cells are wilting, their stomata close to reduce transpiration. Therefore, long periods of light, heat or dry air could result in the stomata closing and decreasing the rate of transpiration.

Potometers are used in transpiration investigations. They do not actually measure the rate of traspiration but the rate of water uptake by the cut stem.

Adaptations to habitats

Xerophytes – adapted to dry habitat
Halophytes – adapted to salty habitat
Hydrophytes – adapted to freshwater habitat
Mesophytes – adapted to habitat with adequate water

Adaptations of xerophytes: